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• # Natural Convection Across a Sphere

Discussion in 'Calculations' started by rogue909, Oct 28, 2014.

1. ### rogue909New Member

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So I am working on a laboratory experiment which entails taking a heated aluminum sphere and placing it in a wind tunnel. The temperature is then measured as a function of time and from there we are to calculate the heat transfer coefficient (h).
All of the data with regards to the sphere is known (temperature, material, geometric) and all of the data with regards to the air is known, or can be calculated, (temperature, Pr, pressure).
The trials are repeated for velocities of v=0.4.8,12. From the data the experimental heat transfer coefficient is calculated. The next portion of the lab is to calculate the data with regards to the theoretical heat transfer coefficient.

For the values where v=/=0 I found the formula
Nu=2+.6(Re)[SUP]1/2[/SUP](Pr)[SUP]1/3
[/SUP]From the Nu number it is pretty much algebra from the definition to determine h
Nu=(h*k)/D

Now, scanning through my heat transfer textbook, I am unable to find the correct formula with regards to when the velocity of the air is equal to 0. The book discusses the condition and states that the above equation fails when Re->0 but doesn't discuss what to do when that happens.

I tried entering 2 as the value of Nu (with little hopes that it would work) but got an theoretical coefficient that was way out of range.

Is there another equation that this book is not providing? Or should I be treating this as another scenario?

Any and all help would be greatly appreciated ^_^

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